Effect of cooling gas on the thermal lens effect of the focusing lens for a high-power CO2 laser

摘要

This paper studies the influence of the cooling gas on the thermal lens effect (TLE) theoretically. It provides a method to calculate the influence of the cooling gas on the temperature of the focusing lens and presents a parameter called relax time of the temperature change (RTTC) of the focusing lens to characterize the temperature change. It studies the change of the laser power density on the surface of workpiece with the time or the cooling gas rate. Cooling gas can reduce the temperature of the lens and make it stable more rapidly. Increasing the flow rate of cooling gas can reduce the TLE greatly, however, it has a limitation. Beyond this limitation, increasing the rate leads to less reducing of the TLE and the RTTC. Double sides cooling is much more efficient than the single side one. Besides, increase of the diameter of the lens can also reduce the RTTC. It also shows that TLE is caused mainly by the change of refractive index of the lens with temperature.